sim: add testing of ECIES-P256 images

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/mcuboot-sys/Cargo.toml b/sim/mcuboot-sys/Cargo.toml
index d20a169..d161eb3 100644
--- a/sim/mcuboot-sys/Cargo.toml
+++ b/sim/mcuboot-sys/Cargo.toml
@@ -36,6 +36,9 @@
 # Encrypt image in the secondary slot using AES-KW-128
 enc-kw = []
 
+# Encrypt image in the secondary slot using ECIES-P256
+enc-ec256 = []
+
 # Allow bootstrapping an empty/invalid primary slot from a valid secondary slot
 bootstrap = []